PV clearence, will I have enough?

[bgjohnson]

well I've pretty much decided on the B Cam from Ford, and I want to go with 1.7 RR, and I think the total lift is about .512. Now I'm pretty sure it would fit with stock ported heads, but I have powerheads that have 1.60 exhaust and 1.90 intake, will it clear those valves, and how would I go about measuring the clearence if I had to? thanks

chris

 

[giddyup306]

well I've pretty much decided on the B Cam from Ford, and I want to go with 1.7 RR, and I think the total lift is about .512. Now I'm pretty sure it would fit with stock ported heads, but I have powerheads that have 1.60 exhaust and 1.90 intake, will it clear those valves, and how would I go about measuring the clearence if I had to? thanks

chris

I think you should be ok BUT check your clearances anyways. You'll need at least 2.54mm p/v clearance. To check the clearance stick clay or play dough on the piston torque down the head bar the engine tear it off and measure the clay with a depth mic. BTW it's usually duration not lift that will cause p/v problems.

 

[bgjohnson]

is the measurement with or without the head gasket?, anybody else have a B cam with these size valves?

 

[65ShelbyClone]

To get actual working measurements you use a head gasket, but I didnt so I would get a worst case scenario. I have a Crane 2031 (.513 in, .529 ex) in my new engine with Edelbrock Performer 60379 heads (1.90/1.60) and no, they did not clear on either valve. I had between .010" and .025" of space before clearancing and recommended minimums are .080 in/.100 ex. Also, dont use Play-Doh because it actually has some elasticity and wont give totally accurate results. Non-drying modeling clay is best.

 

[Michael Yount]

Here's a link on how to measure another way if you like:

http://www.geocities.com/jjonibones/PVC.html

 

[bgjohnson]

so what did you do since they didn't clear the valves? should i go with 1.6 RR instead of 1.7rr, what would the total lift be then? here are the specs


Small Ford: 1985-95 302 (with roller tappet cam), 1994-95 351W (with factory roller cam when used with mass air EFI), 284/284 degrees duration, hydraulic roller camshaft

Advertised duration: 284 intake/284 exhaust
Duration at .050 in. cam lift: 224 intake/224 exhaust
Gross valve lift: .480 in. intake/.480 in. exhaust
Lobe separation: 107 degrees intake/117 degrees exhaust
Peak horsepower rpm: 5,100
Peak torque rpm: 3,300

 

[Michael Yount]

bgjohnson - I think he mentioned clearancing in his post - I'm assuming he clearanced the pistons by enlarging the reliefs. Switching the rocker ratio changes the peak lift by the ratio of the rockers - 1.7/1.6; but peak lift isn't the problem. Read the link I posted above and you'll understand better what the issue is. A rocker ratio change will only gain you a few thousandths of extra clearance. If you're way off (as he was above - he had to go from .01 to .08, and .025 to .100) you can clearance the pistons or go with a cam that opens the intake valve later and/or closes the exhaust valve earlier - that means less duration. Peak lift doesn't matter in p to v clearance.

 

[bgjohnson]

but the bigger valves (1.60,1.90) do make a difference right?, because I know stock size valves usually don't have a problem, but I'm wonderin how much these bigger valves will hurt me in P to V clearence.

 

[65ShelbyClone]

Bigger valves make all the difference in the world. Thats why every head I've seen with 2.02 intake valves has a note saying it wont work with factory pistons. The problem with my piston/head combo is I have Speedpro flat tops with valve reliefs. They have no dish to them at all, unlike stock which has a slight dish. All things being equal, this puts the top of the piston closer to the valves. My problem, however, was not vertical clearance. It was the valve edge being too close to the crescent part of the relief (too big a valve for the pistons/cam). I used a small air grinder to open up the reliefs. Its not the best way, but I doubt it threw the balance off much.

Just a side note - I got my entire shortblock long before I got my heads and originally planned on using ported E7s or at most GT40s. I doubt an E cam would have given me grief, which was one of my options.

 

[Michael Yount]

bgjohnson - Shelby's right on; of course the bigger valves reduce clearance. That combined with a cam that has more duration than stock is what puts you in a position of having to measure to see what your clearance is. Once you've done that, then you'll know whether you have to do anything or not.

 

[bgjohnson]

ok. this kinda stuff just really worries me. So if the clearence is too little I can actually take my dremel and make the stock piston valve reliefs bigger?

 

[Michael Yount]

There are two ways the clearance can be too small. If the valve head diameter allows it to actually fit within the relief, but the edge of the valve head is too close to the relief, then you can increase the diameter of the relief to create the clearance you need. This may also work if the diameter of the valve is larger than the relief, such that it won't fit into the relief at all.

If the valve fits within the relief, but the relief isn't deep enough to provide enough clearance, then the reliefs must be made deeper. If that's the case, that's pretty difficult to do well with a Dremel tool. If the reliefs have to be deepened, then most folks either have a machine shop cut the pistons, or they use a tool/cutter that's designed to be driven by a drill through the valve guide to cut the relief deeper. I know that Isky sells such a relief cutter - you might be able to find it online.

At some point, however, you'll have to measure to know what you may, or may not, have to do.

 

[bgjohnson]

thanks for all the good info. I'll keep my fingers crossed.